Satellites have a large number of wires and harnesses (cables) associated with them. These cables are generally composed of an inner silver-coated copper wire conductor, a central dielectric insulator layer, an outer silver-coated copper shield, and an external dielectric jacket. When these types of cables are exposed to transient X-rays and gamma-rays from a nuclear explosion, that is high-energy photons, large numbers of free electrons will be produced by the inner conductors and outer shield of the prior art cable. Many of these free electrons make their way to many electron traps in the dielectric at both the conductor-insulator interface and at the shield-insulator interface. Most of these latter, previously free, electrons will be stored, that is trapped, in electron traps of the cable insulators.
Trapped electrons occur as a result of excess free electrons transferring from the metal, namely conductors and shield, to the dielectric layer, that is insulator. The presence of gaps and spaces between the metal and the dielectric layer then prevent these excess electrons in the dielectric layer from returning to the metal to recombine with positive ions, namely holes, and hence they are trapped in the dielectric layer.
The spatial distribution of trap-sites and the differential number of trapped negative charges, at or near the shield-dielectric interface versus the negative charges at or near the conductor-dielectric interface, induce an electric field, namely an electromotive force, that acts on other electrons in the inner conductors and the outer shield. A pulse of electrical current called Systems-Generated-Electromagnetic Pulse (“SGEMP”) will automatically flow through any conductive path from the inner conductors to the shield, or from the shield to the inner conductor, to balance the surge of displaced charges and to eliminate the electric field. When such a cable interconnects electronic equipment, a transient SGEMP current pulse is said to flow through the circuits that are the conductive pathways inside the electronic equipment between the shield and the conductors. An SGEMP current pulse, of either positive or negative polarity, can damage sensitive electronic components along its path.
X-rays and gamma-rays cause electrons to be displaced from the shield braid and from the conductors. Depending on the particular cable design, material geometry, gaps, radiation attenuation through materials, and the type of materials involved, more electrons are generally trapped at one interface than the other. This imperfect matching, of the forward-emitted shield wire electrons versus the reverse-emitted conductor core wire electrons, causes a charge imbalance. A resulting replacement current flows from the shield to the core wire, or from the core wire to the shield, through interconnected electronic circuits. Such transient negative or positive current, passing through electrical circuits, can cause upset or damage to sensitive electronic components inside the electronic box or equipment.
Classically, the problems associated with SGEMP current in cables are solved by the use of terminal protection, that is protection placed between the cable and the sensitive electronic circuits or equipment. This is done in order to clamp the voltage and shunt or limit the current to prevent damage to the sensitive interface circuits. Classic current shunting utilizes discreet voltage clamps, such as zener diodes or some other type of clamping diode, or filter capacitors that may be positioned inside or outside the electronic box or equipment. The SGEMP current can also be limited by adding series, current limiting resistance.
As technology has improved over time, speeds associated with data transmission and communications have increased. The use of classic terminal protection can adversely impact the signal integrity of high-speed digital signals. The excess loading caused by a discrete clamping diode or the mismatch caused by an added limiting resistor can degrade signal integrity. An alternative to the use of terminal protection that has been used in the past is to reduce cable SGEMP current by utilizing low SGEMP, radiation-hardened electrical cables such as that disclosed in U.S. Pat. No. 6,093,893. The electrical cable disclosed in U.S. Pat. No. 6,093,893 achieves radiation-hardness by the insertion of low-Z trapped-electron reducers. The trapped-electron reducers reduce the emission of electrons between the high-Z metallic conductor and insulator, and the emission of electrons between the high-Z metallic shield and insulator when the cable is irradiated by high energy photons. The trapped-electron reducers minimize gaps, which reduce the electron range to trap-sites and enhance charge recombination. In addition, radiation shielding can be used, reducing the X-rays and thus the SGEMP current.
The use of radiation hardened cables on a satellite can be quite costly and radiation shielding exceedingly heavy. There is a need for a device or circuit configuration that allows for the use of standard cables on satellites and other equipment having high-speed digital interfaces, whereby a shunt path is provided for the transient SGEMP current pulse without adversely effecting signal integrity.